1. Field of the Invention
The present invention relates to an optical transmitter circuit, for use in the field of optical communications, including a circuit capable of driving a light emitting element at a high speed.
2. Description of the Background Art
A commonly known type of a driving circuit for driving, at a high speed, a light emitting element (e.g., an LED) whose response speed is relatively slow employs a peaking technique. With the peaking technique, an instantaneous current (hereinafter referred to as a “peaking current”) is given to a light emitting element so as to force the light emitting element to respond at a high speed. FIG. 20 shows an exemplary configuration of a common conventional light emitting element driving circuit using a peaking technique. FIG. 21 shows waveform diagrams illustrating an operation of the conventional light emitting element driving circuit shown in FIG. 20.
The conventional light emitting element driving circuit shown in FIG. 20 includes a light emitting element 101, a peaking current generating section 102, and a light emitting element driving section 103. A digital signal S (the waveform (a) of FIG. 21) is inputted to the light emitting element driving section 103. The peaking current generating section 102 generates a spire-shaped peaking current P (the waveform (b) of FIG. 21) at the rising and falling edges of the digital signal S. The light emitting element driving section 103 receives the digital signal S and the peaking current P, and outputs a driving current D (the waveform (c) of FIG. 21) whose waveform is obtained by combining together an amplitude current according to the amplitude of the digital signal S and the peaking current P. The light emitting element 101 receives the driving current D, and outputs an optical signal (the waveform (d) of FIG. 21) whose waveform substantially matches that of the digital signal S. This is how it is possible to realize a high speed response of the light emitting element 101.
However, the response speed that can be realized with the conventional light emitting element driving circuit described above is on the order of Mbps at best. Realizing a response speed on the order of 100 Mbps or more requires the use of a very large peaking current P, which causes clipping at the falling edge in the light emitting element driving section 103. Therefore, the light emitting element 101 cannot be operated at a high speed. A possible solution to this problem is to increase the DC current through the light emitting element driving section 103 so as to prevent the clipping at the falling edge. However, the solution has problems such as an increase in the power consumption, and deterioration in the extinction ratio, which is the ratio between the high level and the low level of the digital signal. In worst cases, the guaranteed range may be exceeded, and the light emitting element 101 may break down.
A technique for solving such a problem is proposed in a patent document (Japanese Laid-Open Patent Publication No. 2002-64433, FIG. 1), etc. FIG. 22 shows an exemplary configuration of the conventional light emitting element driving circuit disclosed in this patent document. FIG. 23 shows waveform diagrams illustrating an operation of the conventional light emitting element driving circuit shown in FIG. 22.
As compared with the conventional light emitting element driving circuit shown in FIG. 20, the conventional light emitting element driving circuit shown in FIG. 22 further includes a discharge circuit 104 for pulling a portion of the driving current D flowing into the light emitting element 101. The peaking current generating section 102 generates a large peaking current P (the waveform (b) of FIG. 23). The light emitting element driving section 103 receives the digital signal S (the waveform (a) of FIG. 23) and the peaking current, and outputs the driving current D (the waveform (c) of FIG. 23) whose waveform is obtained by combining together an amplitude current of the digital signal S and the peaking current P, i.e., whose waveform is such that the amount of clipping at the falling edge is reduced as much as possible. The light emitting element 101 receives a current D′ (the waveform (d) of FIG. 23), which is the remainder after pulling a current from the driving current D by the discharge circuit 104, and outputs an optical signal (the waveform (e) of FIG. 23). With this configuration, it is possible to reduce the DC current flowing through the light emitting element 101 to improve the extinction ratio, and the light emitting element 101 can be operated with an amount of current within the guaranteed range.
However, with the conventional light emitting element driving circuit shown in FIG. 22, it is necessary to increase the driving current D of the light emitting element 101 by the amount of current to be pulled by the discharge circuit 104, thereby increasing the power consumption of the circuit.
It is necessary to increase the driving current D so as to prevent clipping, and it is necessary to provide a very large current, which makes the circuit scale impractically large.
Moreover, instances of clipping include those occurring at a transistor of the light emitting element driving section 103 and those occurring at the light emitting element 101. With the conventional light emitting element driving circuit, it is possible to improve those occurring at the light emitting element 101 but not those occurring at a transistor. Therefore, the falling edge in the waveform of the optical output of the light emitting element 101 becomes deteriorated, as shown in the waveform (e) of FIG. 23.